PEOCEEDINGS OP THE MALACOIOGICAL SOCIETY. 109 



In the discussion which ensued, 



Dr. J. W. Gregory stated his belief that Tanganyika was the oldest 

 of the great African lakes. He supposed that this halolimnic fauna had 

 occurred in the other lakes, it being easy to imagine this fauna to have 

 been exterminated in them by an application of the physical changes 

 now observed to be taking place in certain of them, as for instance Lake 

 Stephanie, which, since its discovery, had been observed to be at one time 

 very salt and at another time very fresh. If such great changes could 

 occur in one lake within a period of about seven years, it was easy to 

 understand how a group of animals could have been exterminated from it. 

 If, however, the whole facies of the fauna could be shown to be of 

 a marine type, then perhaps it would be justifiable to regard this halo- 

 limnic fauna of Lake Tanganyika as a marine derivative. 



Dr. W. T. Blanford said that one fact appeared to be most conspicuous, 

 viz., that this fauna was a very old one, especially when it appeared that 

 these forms commonly showed afl&nities between several very different 

 marine forms, but did not directly resemble any individual living genus. 

 Undoubted marine faunas found in inland waters did not difier from those 

 of adjacent seas, e.g. the Caspian and the Black Sea. If Tanganyika had 

 been recently separated from the sea, it ought to be possible to find the 

 channel of communication, which at present was not the case : the Congo 

 did not appear to represent such a channel, since it seemed to have 

 only recently opened out ; neither did the Eift Valley, which had been 

 too recently formed. 



Mr. W. H. Hudleston said that the comparison which had been instituted 

 by Mr. Moore between these halolimnic gastropods and certain Jurassic 

 forms, appeared to him perfectly justifiable, Bathanalia being in his 

 opinion generically, if not specifically, identical with a species of Amherlya 

 from the Inferior Oolite. The same was true of the genera Paramelania 

 and JVassopsis, which were almost indistinguishable from the Jurassic 

 Pwpurina ; the halolimnic form Lim'notrochus also closely resembled the 

 Jurassic genus Onustus. He, however, drew attention to the fact that 

 none of these Jurassic forms had a very wide range in time ; Purpurina, 

 which lasted longer than the others, apparently died out in the Callovian. 



The following gentlemen also took part in the discussion : Dr. H. 

 Woodward, H. W. Burrows, J. C. MelviU, S. I. da Costa, M. F. 

 Woodward, and Lieut.-Col. Godwin-Austen. 



Mr. J. C. Melvill exhibited a great variety of specimens of PlanorMs 

 spirorhis, Linn., which had been collected near Tenby, Pembrokeshire, 

 in almost stagnant water, by Mr. A. Stubbs. By far the greater 

 number were distorted, some assuming scalariform, some an evolute, 

 others a turbinate shape. Assuming the species of this genus to be 

 sinistral, it was astonishing to fi.nd but few monstrosities of the 

 scalariform type possessing other than a dextral torsion. It is 

 imagined that the aberration of foi'm occurs most frequently in stagnant 

 ditches or ponds choked with ConfervaB, or with Duck-weed, so that 

 the mollusc is thus able more easily to make progress through such 

 obstruction. 



Mr. E. R. Sykes exhibited one of Dr. Menke's exchange lists, 

 which he believed to be the earliest exchange list extant. 



Mr. Gude exhibited photographs of the types of Plectopylis anguina, 

 Gould, P. repercussa, Gould, and P. refuga, G.-Aust., in the I^ew York 



